Osteoarthritis (OA) is a painful and debilitating disease of the synovial joints, affecting an estimated 21 million people in the United States. There is increasing evidence that local and systemic inflammatory cytokines such as interleukin 1 (IL-1) and inflammatory mediators such as free fatty acids, nitric oxide, or prostaglandins play a major role in OA pathogenesis and pain. Additionally, biomechanical loading plays an important role in the normal homeostatic maintenance of the cartilage extracellular matrix, and under abnormal conditions, mechanical stress may be a significant factor in the initiation and progression of OA. Our governing hypothesis is that obesity causes OA through synergistic interactions of dietary and systemic pro-inflammatory mediators, cytokines, and mechanical stress acting on the chondrocytes. The goal of this project is to examine the influence of dietary fatty acids on obesity-associated OA in mice, and to examine their interaction with altered biomechanical and pro-inflammatory cytokines using various in vivo and in vitro models. We propose that low-grade chronic systemic inflammation due to obesity or pro-inflammatory fatty acids in the diet acts in synergy with local inflammatory cytokines or altered mechanical loading (due to obesity or joint instability) to promote a state of inflammation and matrix degradation in the articular cartilage. We will pursue the following aims: In Aim 1, we will examine the role of a high-fat (lard-based) diet in the development of OA in a leptin-receptor deficient mouse (db/db), and we will also measure osteoarthritic changes in diet-induced obese mice fed high-fat diets high in saturated and monounsaturated fatty acids, or omega-3 or omega-6 poly-unsaturated fatty acids. In Aim 2, we will examine the effects of obesity (via high-fat diet or leptin deficiency) on the progression of OA in a destabilized medial meniscus model of mouse OA. In Aim 3, we will use controlled in vitro models of cartilage explant loading to examine the effects of mechanical stress in combination with pro-inflammatory cytokines (e.g., IL-1, leptin, TNF-a) and fatty acids on the anabolic and catabolic activities of the chondrocytes, as measured by biomarker production, real-time PCR measurements of mRNA transcription, and protein synthesis of collagen II and aggrecan. Detailed studies of the interactions between specific biomechanical factors, pro-inflammatory mediators, and tissue metabolism in articular cartilage will improve our understanding of the pathology of the OA, particularly as it relates in vivo to biomechanical factors such as obesity, injury, or weight loss. The results of this study will provide new insights into key elements of the pathogenesis of OA, and ultimately could lead to new treatments that exploit mechanical, psychosocial, and biochemical therapies to prevent disease.